From what I've read, when you grow oyster mushrooms you can either sterilize or pasteurize the substrate, and pasteurization is supposed to be better because it preserves micro organisms that help reduce contaminants.

Recommendations for pasteurization seem to require heating for a long time, why is this - don't things die quickly in heat ?

I didn't have a thermometer to get the proper temperature so I decided to sterilize instead.

I experimented with microwaving my substrate in a jar to sterilize it, which people I've read on forums say isn't a good way to sterilize.

If I microwave substrate until it's sizzling, which I did, what is going to survive ? Why isn't this a sure fire way to sterilize anything ?


3 Answers 3


The reason that pasteurization is used when growing oyster mushrooms is that the media (usually straw) doesn't support contamination as easily as grain media since it's not as rich in nutrients. This means it's also much forgiving in terms of sterile technique when innoculating the media. Grain media readily contaminates in the home so needs to be sterilized.

The reason you have to keep the media heated for such a long time to pasteurize is that the media is usually quite bulky and you need to be sure that the heat is penetetrating the entire bag of media. You can see that pasteurizing milk takes 30 mins with shorter times depending on technique.

A microwave can't reach temperatures much higher than 100 deg C (because it's agitating water molecules) and therefore can not sterilize. If those water molecules are not contained, they turn into steam and leave the substance. If contained, such as in an egg, they tend to explode!

The substance to be heated also needs water content and fungal spores have a low water content making them hard to heat with microwaves.

A pressure cooker is thus pressurized to 15 psi to reach a sterilizing temperature of 121 deg C, and the 90 mins duration for jars is to make sure that that temperature reaches evenly throughout the jars. Bags of substrate such as wood chips have to be heated for more than 2 hours.

Note that straw can be pasteurized without heat using hydrated lime or other alkali techniques, and it's possible that a microwave could be used if you did fractional microwaving where you heated, cooled and allowed the spores to germinate once they had become hydrated, and then heated again in a number of cycles. But this sounds very painful.

  • I see. I was using coffee grinds and cardboard and just microwaving it to sizzling point for a while. I figured if it was good enough for chicken it's good enough for this. That's a good point about spores being dry, though I thought they would still be getting heated, after all even the jar I used got really hot. So, digging further, there are organisms which can survive 100C ? Commented Jan 21, 2018 at 20:06
  • Coffee grounds need to be sterilized as it's a high nutrient substrate but usually when you get them freshly made they're sterile as the beans were roasted. The trichoderma spores survive 100 deg C as do other viruses etc. That's why medical instruments need to be autoclaved which is just a fancy pressure cooker. Commented Jan 21, 2018 at 20:13
  • You don't need to sterilize cardboard. It's a poor substrate for trichoderma so just soaking overnight in boiling water that's allowed to cool is sufficient. Commented Jan 21, 2018 at 20:16
  • Experience seems to be bearing all this out. All my coffee grind stuff is going green, but I tried some with just cardboard and that seems to be doing much better. Thanks for the info. Would it be safer to grow on cardboard first, then throw some coffee in when it's established ? Commented Jan 21, 2018 at 20:43
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    For thin objects (e.g. herbaria), it was noticed that microwaving damage DNA, so it should be effective to sterilize, but a very thin substrate at a time. Commented Jan 22, 2018 at 7:56

Pasteurizing coffee grounds in a microwave works.

I used 7.5 minutes on three litre buckets of coffee grounds, with lid fastened when it cools to just warm. When left unopened no mould grew, then after opening I would sometimes get Trichoderma (green mould).

Of course this process is not "sterilizing", but for this purpose killing off the mould spores is all that is required - the survival of various bacteria is not a problem.

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A microwave can definitely sterilize and will go above 100C. The fact that water boils at 100c does not stop the microwave from inducing continued energy on the water molecules though they will be in gas format as steam. 100c is also sufficient to kill all life forms if given enough time. The encapsulated spires are somewhat protected but once the proteins needed for life get denatured the mechanisms of life are broken.

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